The present invention relates to a washing machine using a three-phase induction motor, or more in particular to a washing machine estimating the amount of laundry and detecting an unbalanced amount caused by uneven distribution of laundry in a rotatable drum.
The motor for driving the agitator of the conventional washing machine is a single-phase induction motor in most cases. The single-phase induction motor drives the agitator by rotating at a constant rotational angular velocity determined by the power frequency and the reduction ratio of the reduction gear such as gears and belt. The single-phase induction motor has so small a torque in the range of low speeds that the amount of laundry that can be washed at a time and the washing system are limited.
As a means for solving this problem, JP-A-7-255988 proposes a method for varying the rotational speed of the agitator using a DC brushless motor and an invertor circuit.
On the other hand, JP-A-6-351292 proposes a method for varying the rotational speed of a three-phase induction motor by V/f control using an invertor circuit.
A conventional washing machine estimating the amount of laundry in the rotatable drum is disclosed, for example, in JP-A-61-8094. In this prior art, the motor for driving the rotatable drum stops being driven at the time point when the rotational speed of the motor reaches a predetermined level wherein the amount of laundry is estimated based on the time from deenergization to of the motor to subsequent stop of the motor under rotation with inertia. On the other hand, JP-A-6-71085 proposes a method in which the voltage applied to the motor is maintained constant and the amount of laundry is estimated from the change in angular velocity information detected by an angular velocity sensor mounted on the motor. Further, JP-A-5-3990 discloses a method of estimating the amount of laundry from the current supplied to the motor.
In a conventional washing machine disclosed in JP-A-3-70596 which detects an unbalanced amount due to the uneven distribution of laundry in the rotatable drum, a sensor for detecting the vibrations is mounted in the rotatable drum to detect an unbalance. Also, JP-A-5-103895 discloses a method for detecting an unbalanced state from the angular velocity information detected by an angular velocity sensor mounted on the motor. Further, JP-A-9-290089 discloses a method using a position sensor for detecting an unbalanced state of the rotatable drum and measuring the position of unevenly distributed laundry in the rotatable drum from the variations in the current flowing in the motor.
The conventional method using a DC brushless motor has encountered a problem of high cost of the DC brushless motor in comparison with an induction motor.
In the conventional method of controlling the angular velocity of a three-phase induction motor by invertor, a constant ratio V/f between the applied voltage and frequency of the motor must be maintained. However, the controllability of the three-phase induction motor is inferior to that of the DC motor. Especially, it is not easy to produce a large torque in low-speed ranges. Therefore the problem is that a large amount of laundry cannot be easily washed at a time.
The method of estimating the amount of laundry such as the weight from the time during which the drive motor rotates under inertia has a low accuracy. Also, the method of estimating the amount of laundry based on the rotational angular velocity of the motor requires mounting of an angular velocity sensor on the motor.
The above-mentioned methods of estimating the amount such as the weight of laundry in the rotatable drum has the problem of a low estimation accuracy, regardless of whether the amount of laundry is estimated from the change in the rotational angular velocity of the motor under a predetermined voltage applied thereto, or from the current supplied to the motor.
The method for detecting the unbalanced amount in the rotatable drum, requires mounting of a sensor for detecting vibrations. On the other hand, detection of an unbalanced amount accurately from the change in rotational angular velocity requires a highly accurate angular velocity sensor, and therefore is expensive. Further, the method of detecting an unbalanced amount from the variations in the current supplied to an induction motor, poses the problem of a low detection accuracy.